Field of the Invention
The present invention relates to a robot apparatus in which a wire member that transmits a driving signal and/or a control signal to each unit of a robot arm therethrough is arranged along a plurality of links which constitute a robot arm; and a method for controlling the same.
Description of the Related Art
In recent years, in a robot apparatus, for instance, in a multi-axis and multi-joint robot in which a joint is rotated and constrained, a multi-joint robot has received attention which arranges a torque sensor for measuring a torque that a driving source exerts on a link, in a joint, and controls the torque (force) which occurs in the joint. By arranging the torque sensor in the joint, it has become easy for the multi-joint robot to control the force occurring in the joint thereof and to control a load and/or a force which an end effector arranged in the front end of the robot arm exerts on a component.
For the present, the robot apparatus that has the torque sensor and can control the torque is often used for the assembly in which a load of several hundreds to several kilograms is exerted on a component, such as an assembly of an engine part of an automobile. On the other hand, the robot apparatus is not used so often for an assembly of such a very light load that the load or the force to be exerted on the component at the time of assembly is approximately several grams, for instance, for an operation of a tiny component of several grams, a film, a sheet and the like. One of the reasons is in such a point that the present multi-joint robot does not control the force (torque) with so high precision, and still cannot attain a sufficient precision range to carry out an assembly of such a range that a load which an end effector of the front end of the robot arm exerts on a component is approximately several grams.
For instance, a robot apparatus needs a transmission member for transmitting a driving signal, for controlling an actuator that drives the joint, between each of the units of the robot arm, for instance, between the actuators. These driving signals include electric signals belonging to a category of control information such as a control signal, and in addition, signals belonging to a category of a driving energy such as a driving electric power and a pressure signal of an oil pressure and/or an air pressure. In the present specification, the above-described signal such as the electric signal and the pressure signal for transmitting the control information and the driving energy, and a signal for controlling the actuator of each of the units of the robot arm are referred to as the “driving signal”.
In a robot which uses, for instance, a rotational driving source such as a motor, as a driving source (actuator) that drives the joint and the end effector, a wire member such as an electric wire (cable) is used as a transmission member which transmits the control signal and the driving power to this motor and the driving circuit. In addition, when the robot uses the actuator which uses the oil pressure, the air pressure and the like for driving the joint and the end effector, a wire member like a pressure pipe which is structured of a flexible material, for instance, such as rubber, is occasionally used as the transmission member, in order to transmit the driving signal (energy).
In addition, as for the wire member of the cable and the pressure pipe as described in the above, there are many structures in which only one wire is not laid, but a wire member having a harness structure is laid, in which wire members, for instance, corresponding to a plurality of joints of the robot arm are bundled. In addition, such a harness occasionally accommodates wire members therein for an encoder which detects the operation of the actuator that drives the joint, and for feeding a signal of the above-described torque sensor back to a control system. In addition, a gripping apparatus, for instance, a hand or a gripper which acts as the end effector (or tool) is occasionally mounted on the front end of the robot arm. It is necessary to input/output (transmit) the above-described driving signal also to such an end effector (or tool). In this case, the above-described harness of the wire members occasionally accommodates a wire member therein through which inputs/outputs the driving signal to/from the end effector (or tool).
The above-described wire member (or harness thereof) is laid in the inside or outside of the robot arm, and in this case, is installed so as to straddle each of the joints of the robot apparatus. Accordingly, the wire member which transmits the driving signal therethrough is deformed when the joint of the robot apparatus is rotationally operated (similarly also when being translationally operated), and generates a reactive force according to the deformation.
On the other hand, when the torque (force) sensor is provided on the joint which supports the link (or end effector unit) and the torque (force) is controlled, as in the above description, it is desirable to measure fundamentally only the torque occurring in the joint with the torque sensor, and to feed the measured torque back to the torque control system. However, the reactive force which is generated by the deformation of the wire member that transmits the driving signal to the above-described joint (and/or end effector) which is associated with the operation of the robot apparatus gives influence on a measured value of the torque sensor that is arranged in the joint, as a measurement error.
The reactive force generated when the harness of the wire members of various cables and pressure pipes is deformed results in becoming a unit of several hundred grams, though varying depending on the structure and the scale of the robot apparatus. Accordingly, when the force (or torque) is controlled with a precision of several grams, there are problems that such a reactive force becomes an extremely large disturbance, and gives influence on the precision of the control.
With respect to the above-described problem, in order to enhance force controllability, for instance, a constitution is proposed (see Japanese Patent Application Laid-Open No. 2012-218104.) that includes: previously preparing a relationship between a rotation angle of the joint and a cable reactive force, which has been measured beforehand, as a table; estimating a force occurring in the joint of the robot apparatus; and controlling each of the joints.
However, in the above-described Japanese Patent Application Laid-Open No. 2012-218104, a table of the cable reactive force is formed and used which has been measured beforehand, and accordingly the cable installed in the actual robot apparatus does not necessarily generate the reactive force as shown in the table, in regard to a particular joint action. This is because the deformation of the wire member, for instance, such as the cable does not show reproducibility, and because of this, probably, the reactive force generated by the deformation also does not show reproducibility. Secondly, the wire member cannot be always arranged in the state equivalent to that in the preliminary measurement, and the same reactive force is not actually generated by the same joint action as in the preliminary measurement, because of such a problem of installation precision.
Accordingly, in the technology of Japanese Patent Application Laid-Open No. 2012-218104, the reactive force of the wire member is considered in the control, but still there is a problem that the force (torque) cannot be controlled with sufficient precision. In the conveyance of an article and an assembly of an article having a weight of several kilograms, such lowering of a control performance of an infinitesimal force as described above does not cause the problem, but when a component having a very light weight of several grams is assembled through a force control, the lowering of force controllability originating in the cable reactive force becomes unable to be neglected. When a range of loads which a robot deals with is large, there is a possibility that the technology of Japanese Patent Application Laid-Open No. 2012-218104 can be practically used. However, in the control for the force (torque) particularly of the order of several grams as has been described above, even the technology in Japanese Patent Application Laid-Open No. 2012-218104 cannot attain a sufficient precision, and there is a possibility of showing lowered force controllability.
The reason why a general-purpose multi-joint robot apparatus has not been conventionally positively used for the assembly of a very light load is attributable to the problem as described above. In addition, in the assembly of an article having a very light weight of several grams, the apparatus has been conventionally used, instead of the robot apparatus, which has a special structure, has a size and a precision range that are sufficiently controlled and is manufactured only for the assembly, in many cases.
On the other hand, it is also anticipated that the manufacture of many industrial products is directed toward the manufacturing of a wide variety of products in small quantities, in future. Then, a circumstance that the dedicated apparatus as described above becomes separately necessary for each of tiny (minute) components can become a significant bottleneck in regard to the achievement of the manufacturing of the wide variety of products in small quantities. If a large number of the tiny (minute) components necessary for the manufacturing of the wide variety of products in small quantities each need the design and manufacture of the above-described dedicated apparatus, such problems possibly occur that a start-up period of a production line is prolonged and a start-up cost increases, because of the necessity.
Then, it is considered that if the general-purpose multi-joint robot apparatus can control the force (torque) with sufficient high precision in consideration of the reactive force of the wire member, the dedicated apparatus is not needed, and the robot apparatus can manufacture a wide variety of products in small quantities with high efficiency in a short period.